1. Field of the Invention
This invention relates generally to a gas supply system at a process gas supply point and in particular to a gas supply system at a process gas supply point that automatically evacuates both the gas supply system and a process line supplied by the gas supply system.
2. Description of Related Art
Many hazardous gasses, e.g., caustic, explosive, or poisonous gasses, are routinely used in industry. For example, in semiconductor processing, hydrogen and silane are routinely used. Typically, the gas is contained in a cylinder 101 (FIG. 1) under pressure. One or more cylinders are contained in a gas cabinet 102. Cylinder 101 is coupled to a process line 104 by a gas supply system 103. The hazardous gas flows from cylinder 101 through gas supply system 103 to process line 104 and in turn through process line 104 to a point of use 105, such as an epitaxial reactor used in semiconductor processing. Typically, point of use 105 is in a special room 106 or facility that is located some distance from gas cabinet 102.
When gas cylinder 101 is empty or nearly empty, gas cylinder 101 is replaced with a new cylinder 107. During the new gas cylinder installation, gas supply system 103 first isolates process line 104 so that contaminants are minimized at point of use 105. Next, gas supply system 103 is purged using a non-hazardous gas. When gas supply system 103 contains only the non-hazardous gas, cylinder 101 is removed and new cylinder 107 installed.
After new cylinder 107 is installed, the process is reversed. The non-hazardous gas in gas supply system 103 is purged using gas from newly installed cylinder 107 and then process line 104 is opened so that the gas is available at point of use 105.
One configuration of gas supply system 103 is illustrated in more detail in FIG. 2 as gas supply system 203. During normal operation, gas flows from cylinder 201 through valve 210 and filter 211 to valve 212. Valve 212 is a three-way valve that diverts the flow to regulator 215. Regulator 215 controls the gas flow into process line 204. As is well known to those skilled in the art, regulator 215 allows flow only in the direction from gas cylinder 201 to process line 204 and prevents reverse flow from process line 204 through regulator 215.
To change cylinder 201 with gas supply system 203, the position of valve 212 is changed so that gas flow to regulator 215 is prevented and thereby process line 204 is isolated. Valve 210 is closed to block flow from cylinder 201 and valves 213 and 217 are opened. When valve 217 is open, a vacuum generator supply gas flows through check valve 218 and valve 217 and vacuum generator 216 to purge vent line 221.
As is known to those skilled in the art, when the vacuum generator supply gas flows through vacuum generator 216 a relative low pressure is produced on the line to the vacuum generator from check valve 214. Accordingly, the relatively higher pressure hazardous gas left in gas supply system 203 flows through filter 211, valve 212, valve 213 and flow check valve 214 and is evacuated through purge vent line 221. In this way, the hazardous gas is removed from gas supply system 203 so that cylinder 201 may be replaced without danger of hazardous gas flow through valve 210 if valve 210 should be inadvertently opened. Notice, however, that both regulator 215 and valve 212 prevent evacuation of process line 204 by this method.
Another configuration for gas supply system 103 is illustrated in FIG. 3 as gas supply system 303. In gas supply system 303 during normal operation, gas flows from cylinder 301 through filter 311 and valve 302 to regulator 315. High pressure isolation valve 312 blocks flow to vent line 321 during normal operation. Again, flow through regulator 315 is only in the direction from gas cylinder 301 to process line 304.
In normal operation, gas flow from regulator 315 passes through low pressure isolation valve 323 to process line 304. Low pressure isolation valve 323 blocks gas flow through valves 324 and 325 to vent line 321 during normal operation.
If the regulator 315 must be changed, valve 312 is positioned to connect the high pressure side of regulator 315 to vacuum generator 316 through valve 313 and check valve 314. Similarly, low pressure isolation valve 323 is positioned to isolate process line 304 and connect the low pressure side of regulator 323 to vacuum generator through check valve 325 to vent 321. Vacuum generator 316 functions in a manner identical to vacuum generator 216 (FIG. 2) so that the hazardous gas is evacuated so that regulator 315 can be changed.
In this embodiment to change cylinder 301, low pressure isolation valve 323 is positioned so that flow from regulator 315 passes through valve 324, which is opened, and check valve 325 to vent line 321 and consequently, process line 304 is isolated. A purge gas is introduced through check valve 326 and purge gas control valve 310 to filter 311 and passes through valves 312, 313 and 314 to vacuum generator 316 in vent line 321.
To enhance the evacuation of the purge gas, a vacuum generator supply gas passes through valve 318 and valve 317 and through the venturi in vacuum generator 316. The vacuum generator supply gas flow through the venturi in vacuum generator 316 creates a vacuum that in turn speeds up the evacuation of gas supply system 303 during the purge operation.
In each of the above gas supply systems 103, 203 and 303, the process line gas flow is blocked during operations in which the gas supply system is purged of the hazardous gas. These gas supply systems do not provide an automated means for purging the hazardous gas in process line 304. However, governmental regulations typically require purging the hazardous gas in the process line during or following abnormal events such as power failures, earthquakes, tornadoes, fires, or other natural hazards.
In one system used to purge process line 104 (FIG. 1) of any hazardous material, a valve is supplied in point of use 105 vent line which automatically opens in an emergency. A purge gas is passed through process line 104 to point of use 105 and through the point of use vent. Purging through point of use 105 may introduce contaminants and other problems with the equipment which are undesirable.
Another alternative is to run a second line from point of use 105 back to the vent in gas supply system 103. A valve in process line 104 at point-of-use 105 diverts the flow from point-of-use 105 to the return line during abnormal circumstances. In this case, the purge gas flows through the process line and is diverted by the valve at the point of use to the return line. The purge gas forces the hazardous gas from the process line back through gas supply system 103 to the vent. Unfortunately, the dual piping and the specifications required for hazardous gas piping greatly increase the cost of the system. Accordingly, a low cost and reliable means for purging the process line of hazardous gas during abnormal situations is needed.